The process of laser ablation is known in the art of depositing thin films of material on a substrate and is described, for example, in U.S. Pat. No. 4,701,592 entitled "Laser Assisted Deposition and Annealing." The '592 patent also describes an undesirable side effect of pulsed laser ablation termed "splashing", in which solid particles and molten globules ejected from the surface of the laser target cause poor morphology and low crystalline quality of the deposited film.
Splashing has been attributed to two major causes: subsurface boiling and defoliation. Defoliation is a term used to describe the ejection of weakly bonded irregularities from the rough surface of the laser target. Subsurface boiling can be avoided in most processes because it generally occurs only at very high power densities. Defoliation, however, is a more serious problem because irregular surface features, such as deep craters and needle formations, are inevitably produced when a planar target surface is scanned in a raster pattern by a focused laser beam.
Through experimentation it has been determined that splashing is minimized, and thus smooth films are deposited, when a freshly polished target is used for the first time. However, when a target is scanned repeatedly by a focused laser beam in a raster pattern, the surface of the target becomes cratered and the morphology of the film deposited on the substrate deteriorates. Therefore, it is apparent that undesirable splashing due to defoliation can be minimized by maintaining a smooth target surface during laser ablation.